The present invention relates to subterranean hydrocarbon recovery. More particularly, the present invention relates to methods and compositions for the in-situ thermal stimulation of hydrocarbons using peroxide-generating compounds.
Non-traditional sources of hydrocarbons are playing an increasingly important role in the oil and gas industry. Such non-traditional sources include gas hydrates, heavy oil, bitumen, and coal bed methane. Gas hydrates are a common form of a unique class of chemical compounds known as clathrates, in which a rigid, open network of bonded host molecules enclose, without direct chemical bonding, appropriately sized guest molecules of another substance. In the case of gas hydrates, water acts as the host molecule, enclosing gas molecules such as methane. Recent estimates indicate that gas hydrates, and in particular methane hydrates, may contain more organic carbon than all the world's coal, oil, and non-hydrate natural gas combined; thus, making them an important potential source of energy. Heavy oil (i.e., any petroleum with an API gravity less than 28 degrees) and bitumen (e.g., asphalt and mineral wax) are also important potential energy sources, as the total heavy oil and bitumen reserves of Canada and Venezuela alone are believed to equal the light oil reserves of Saudi Arabia. Similarly, coal bed methane is also becoming an increasingly important energy source, with the total coal bed methane reverses in the United States estimated to be between 400 and 850 trillion cubic feet.
A variety of methods have been employed to facilitate the recovery of these non-traditional sources of hydrocarbons. One method of thermal stimulation common in the recovery of these non-traditional sources involves in-situ combustion wherein oxygen is injected into a reservoir and the hydrocarbons are ignited in a controlled fire, either through spontaneous combustion or by using an ignition source. The heat generated by the burning of heavy hydrocarbons produces hydrocarbon cracking, vaporization of light hydrocarbons, and the generation of water, in addition to the deposition of heavier hydrocarbons known as coke. As the fire moves, the combustion front pushes ahead a mixture of hot combustion gases, steam, and water, which in turn reduces oil viscosity and displaces oil toward production wells.
In-situ thermal stimulation has also been used to recover coal bed methane. Generally, the rate and amount of methane that can be desorbed from materials contained within the physical coal structure is highly sensitive to the in-situ temperature of the coal. The higher the in-situ temperature, the greater the quantity of total methane that can be recovered and the faster the rate at which the recovery can be achieved. Typically, coal seams considered for methane extraction are found at relatively shallow depths, where the desorption isotherm of the methane is limited by the low temperature. In-situ heating increases the desorption rate and the amount of methane recoverable from the coal seam. In-situ heating may also change the physical structure of the coal to enhance the diffusivity and permeability of the coal, allowing more efficient drainage of methane gas from the surrounding volume of coal.
One variation on such in-situ combustion involves injecting hydrogen peroxide, instead of oxygen, into the formation to stimulate the production of hydrocarbons. Inside the formation, hydrogen peroxide decomposes in a highly exothermic reaction to form water and oxygen. The oxygen released by the decomposition of the hydrogen peroxide may then react with hydrocarbons in the formation or with the formation itself, generating carbon dioxide, water, and heat that can be used to reduce oil viscosity and displace oil toward production wells, similar to the in-situ combustion methods described above.
Unfortunately, in-situ thermal stimulation methods using peroxide-generating compounds, such as hydrogen peroxide, have been hampered by the premature decomposition of the compound before it reaches the desired location within the subterranean formation. Attempts to inhibit the decomposition by manipulating the concentration or pH of the peroxide-generating compound have met with limited success and have, in fact, been known to adversely affect the formations into which the compound is injected. Furthermore, safety issues involved with the pumping of peroxide-generating compounds have also limited the widespread application and usage of such compounds in the oilfield.